Electronic displays typically use a light source to shine light onto a display to improve visibility of content on the display. For example, many electronic devices use backlights that light up a display to enable a viewer to see content on the display that would otherwise be difficult to see without the backlights. In another example, reflective displays can use frontlights to improve visibility of content on displays, particularly in low light situations.
Electronic displays can incorporate one or more light guides to direct light from a light source onto or through a display, for example. In some applications, a light source can be relatively small, such as a light-emitting diode (LED). In conventional in-coupling, LEDs are placed close to an edge of a light guide film and light couples in to the light guide film. When the light guide film is made thinner than the light source, the amount of light that can be coupled inside the light guide is greatly reduced, which wastes electrical power.
A light back plane of an LCD display device typically includes a light guide film. Physical dimensions of a light guide film can be described by length A, width B and thickness C, and terms “length,” “width” and “thickness” are relative terms. Generally, length A and width B are much greater than thickness C. A size (e.g., height or width) of a light source is typically an order of magnitude greater than a thickness C of a light guide film. A light source emits light from an area of more than about one square millimeter while a thickness of a light guide film may be less than about 0.5 millimeters. Thus, such a light source can be substantially larger than a height of an edge or end-face of a light guide film. Moreover, a light source typically emits light over a relatively large angular range. Angular distribution of light emitted by an LED can include a cone that subtends a relatively wide angle (e.g., about 30 degrees, in one example implementation). Accordingly, though it may be desirable to inject all light emitted by a light source into a light guide film, a large portion of the light is often not successfully injected into a thin light guide film.